In integrated circuits manufactured with negative substrate biased technology, p-channel devices or MOSFETS are not used to drive data onto output pins. N-channel devices are used for both pull-up and pull-down in the output buffer to prevent the possibility of direct current injection from the output pins to the substrate through the parasitic pnp bipolar junctions of the p-channel transistors. The use of n-channel devices in the output drivers is also advantageous for eliminating the problems of tub-biasing for p-channel pull-up and the hazards of bootstrap circuits.
When an n-channel transistor is used as a pull-up device to drive the output pin, its output can be pulled up to a voltage level that is one threshold voltage level, V.sub.T, less than the supply voltage, V.sub.DD. In applications where the supply voltage, V.sub.DD, is five volts, the output high voltage level, V.sub.OH, can usually attain adequate TTL output level with no difficulty. However, in applications where the supply Voltage V.sub.DD is below five volts or 3.3 volts, for example, the one threshold voltage drop becomes significant, and the resulting output high voltage may be well below acceptable TTL I/O levels.
In particular, in certain 64 Mb dynamic random access memory (DRAM) applications, the supply voltage, V.sub.DD, is 3.3 volts. Referring to FIG. 1, in these DRAM applications, the gate of the n-channel pull-up MOSFET 10 in each output buffer 12 is switched by a high-voltage switching circuitry 14 coupled to a high-voltage pumped supply, V.sub.PP, to be able to attain acceptable high TTL threshold level at the output. In contrast to the supply voltage, V.sub.DD, the high-voltage supply, V.sub.PP, is a limited reservoir of charge generated on-chip. Because each DRAM contains multiple output buffers, and each output buffer draws substantial current from the high-voltage pump as it switches to drive the output pins, the total current drain on the high-voltage pump is significant.
For example, each buffer switching an n-channel pull-up transistor with a 500 .mu.m gate draws an estimated 0.54 mA for a 64 Mb DRAM operating at 20 MHz with V.sub.PP at 5.0 volts. For a DRAM with eight output buffers, the total V.sub.PP current draw is 4.32 mA; for a DRAM with sixteen output buffers, the total is 8.64 mA; and for a DRAM with thirty-two output buffers, the total V.sub.PP current requirement becomes 17.28 mA. As the number of output buffers doubles and quadruples, the requisite V.sub.PP pump becomes prohibitively large and noisy to operate. Therefore, increasing the pump capacity and size are not feasible solutions to an increasing V.sub.PP current demand.
Accordingly, a need has arisen for an output buffer that is able to drive an n-channel MOSFET to acceptable TTL output high voltage levels with substantial low power consumption.